On 26.10.2023 11:46, Xenia Ragiadakou wrote: > On 26/10/23 11:45, Jan Beulich wrote: >> On 26.10.2023 10:34, Xenia Ragiadakou wrote: >>> On 26/10/23 10:35, Jan Beulich wrote: >>>> On 26.10.2023 08:45, Xenia Ragiadakou wrote: >>>>> --- a/xen/arch/x86/hvm/dom0_build.c >>>>> +++ b/xen/arch/x86/hvm/dom0_build.c >>>>> @@ -518,7 +518,7 @@ static paddr_t __init find_memory( >>>>> if ( end <= kernel_start || start >= kernel_end ) >>>>> ; /* No overlap, nothing to do. */ >>>>> /* Deal with the kernel already being loaded in the region. */ >>>>> - else if ( kernel_start - start > end - kernel_end ) >>>>> + else if ( kernel_start + kernel_end > start + end ) >>>> What meaning has the sum of the start and end of either range? I can't >>>> figure how comparing those two values will be generally correct / useful. >>>> If the partial-overlap case needs handling in the first place, I think >>>> new conditionals need adding (and the existing one needs constraining to >>>> "kernel range fully contained") to use >>>> - as before, the larger of the non-overlapping ranges at start and end >>>> if the kernel range is fully contained, >>>> - the tail of the range when the overlap is at the start, >>>> - the head of the range when the overlap is at the end. >>> Yes it is not quite straight forward to understand and is based on the >>> assumption that end > kernel_start and start < kernel_end, due to >>> the first condition failing. >>> >>> Both cases: >>> (start < kernel_start && end < kernel_end) and >>> (kernel_start - start > end - kernel_end) >>> fall into the condition ( kernel_start + kernel_end > start + end ) >>> >>> And both the cases: >>> (start > kernel_start && end > kernel_end) and >>> (end - kernel_end > kernel_start - start) >>> fall into the condition ( kernel_start + kernel_end < start + end ) >>> >>> ... unless of course I miss a case >> Well, mathematically (i.e. ignoring the potential for overflow) the >> original expression and your replacement are identical anyway. But >> overflow needs to be taken into consideration, and hence there is a >> (theoretical only at this point) risk with the replacement expression >> as well. As a result I still think that ... >> >>>> That said, in the "kernel range fully contained" case it may want >>>> considering to use the tail range if it is large enough, rather than >>>> the larger of the two ranges. In fact when switching to that model, we >>>> ought to be able to get away with one less conditional, as then the >>>> "kernel range fully contained" case doesn't need treating specially. >> ... this alternative approach may want considering (provided we need >> to make a change in the first place, which I continue to be >> unconvinced of). > Hmm, I see your point regarding the overflow. > Given that start < kernel_end and end > kernel_start, this could > be resolved by changing the above condition into: > if ( kernel_end - start > end - kernel_start ) > > Would that work for you?
That would look quite a bit more natural, yes. But I don't think it covers all cases: What if the E820 range is a proper sub-range of the kernel one? If we consider kernel range crossing E820 region boundaries, we also need to take that possibility into account, I think. Jan
